With the recent spread of personal computers, word processors, etc., among the general public, magnetic recording media including magnetic disks are required to be of higher quality.
Particularly, there is a growing desire for magnetic disks which can record information at high densities. For the purpose of making magnetic disks for high-density recording, methods are employed, in which ferromagnetic particles having a small particle size are used, ferromagnetic metal particles are used as ferromagnetic particles, or ferromagnetic particles are dispersed more uniformly in magnetic layers. Whichever method is selected, the surfaces of magnetic layers need to be smooth so as to diminish spacing losses caused by the spacing between magnetic head and magnetic layer. However, as a result of a smoother surfaces of magnetic layers, the coefficient of the friction between such magnetic layers during running and parts of recording and reproducing devices have become so high that some problems have become conspicuous. That is, for example, the smooth running of the magnetic disk is often inhibited, the magnetic layers tend to suffer damage , and the start of the running of the magnetic disk requires increased torque.
The above-described problems concerning running durability have become more and more serious for rotary recording media such as floppy disks for use in personal computers, word processors or the like, whose magnetic layers are subjected to many repeated head-on-off operations, especially when such operations are conducted at high temperatures or conducted over a long period of time under condition of cyclically varying temperatures, from high temperatures to low temperatures.
A magnetic disk especially suitable for high-density recording is a magnetic disk comprising a ferromagnetic metal powder and a binder (hereinafter referred to as "metal powder-containing magnetic disk"). Since magnetic disks of this type are excellent in coercive force (Hc), saturated magnetization (Bm) and other magnetic properties, they are hopeful as magnetic recording media suitable for high-density recording, and have been already put to practical use as video floppy disks, etc.
The above-described metal powder-containing magnetic disks also have the above-described various problems accompanying high-density recording. That is, the coefficient of friction between the magnetic layers of these disks during running and parts of recording and reproducing devices tend to increase, and this causes problems on running durability. Specifically, the output becomes low, and the magnetic layers suffer damage leading to the clogging of heads and the peeling of the magnetic layers. Particularly, at a high temperature, since the strength of the magnetic layers becomes low, problems sometimes occur such that the magnetic layers are damaged by the liners in the disk jackets or are peeled from the substrates by magnetic heads.
In order to overcome the above problems, various methods have so far been proposed.
For example, methods for adding lubricating agents to magnetic layers include a method in which a polyethylene glycol ester is added (JP-A-57-3225), a method in which a ketone compound is added (JP-A-57-158026), a method in which a glycerol ester of a fatty acid and a sorbitan ester of a fatty acid are added (JP-A-57-158027), a method in which a fatty acid and a fatty acid ester are added (JP-A-58-130435 and JP-A-60- 5420) and a method in which an alkyl ester of a fatty acid and an alkoxyl ester of a fatty acid are added (JP-A-61-229234 and JP-A-61-230624) (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application"). However, these methods are disadvantageous in that the regulation of the amount of the lubricating agent(s) added is(are) difficult, and that where the lubricating agent1s) is(are) added in amount(s) to produce desired effects, the magnetic layers often develop low mechanical strength, so that their durability is impaired rather than improved, or there are cases where the magnetic layers adhere to magnetic heads.
Other solutions for the problems concerning running durability are methods wherein abrasive materials are added to magnetic layers. Examples of such methods include a method in which .alpha.-ferric oxide is added (JP-A-58-159227), a method in which Al.sub.2 O.sub.3 is added (JP-A-58-171721) and a method in which 0.2 .mu.m or smaller particles having a Mohs' hardness of 6 or more are added (JP-A-61-289528). However, if such an abrasive material is added in an amount to improve running durability, magnetic heads may abrade quickly or the surface properties of the magnetic layers, are impaired resulting in poor electromagnetic characteristics.
As still other solutions, for example, a method in which an abrasive material is added in combination with a fatty acid ester (JP-A-58-189826) and a method in which carbon black is added in combination with silicone oil (JP-A-62-195730) have been proposed.
All of the above methods, however, are defective in that properties other than running durability are adversely affected, or the running durability of the metal powder-containing magnetic disks cannot be improved to a satisfactory extent, although improved to some extent. In particular, the above methods are almost ineffective in eliminating the problem that the ferromagnetic metal powder falls off the magnetic layers to cause the clogging of heads while the magnetic disks are allowed to run at high temperatures, with many repeated head-on-off operations.